How to improve 4 kinds of defects such as cracks and warpage of injection molded products?

Time:2021-01-07 11:43:04 / Popularity: / Source:

One, cracking

It is a common defect in plastic products, main cause is due to stress and deformation. There are mainly residual stress, external stress and stress deformation caused by external environment.

Residual Stress

Cracks caused by residual stress are mainly caused by following three conditions, namely, overfilling, ejection from mold and metal inserts.
Solution can mainly be started in following aspects:
①Since pressure loss of sprue is the smallest, if cracks are mainly generated near sprue, consider switching to multi-point distribution point gate, side gate and shank gate.
②Under premise of ensuring that resin does not decompose or deteriorate, appropriately increasing plasticizing temperature of resin can reduce melt viscosity and improve fluidity. At the same time, it can also reduce injection pressure to reduce stress.
③Under normal circumstances, stress is likely to occur when mold temperature is low, so temperature should be increased appropriately. But when injection speed is higher, even if mold temperature is lower, stress generation can be reduced.
④ Excessive injection and pressure holding time will also produce stress. It is better to shorten it appropriately or perform multiple pressure holding switches.
⑤ Non-crystalline resins, such as AS resin, ABS resin, PMMA resin, etc., are more likely to produce residual stress than crystalline resins such as polyethylene and polyoxymethylene, so attention should be paid.
Ejection from mold:
When demolding and pushing out, due to small demolding inclination, mold cavity and roughness of punch, pushing force is too large, causing stress, sometimes even whitening or cracking occurs around pushing rod. As long as you carefully observe location of cracks, you can determine cause.
Metal inserts:
When inserting metal parts during injection molding, stress is most likely to occur, and cracks are likely to occur after a period of time, which is extremely harmful.
This is mainly due to large difference in thermal expansion coefficients of metal and resin, which generates stress, and over time, stress exceeds strength of gradually degraded resin material to cause cracks. General-purpose polystyrene is basically not suitable for adding inserts, and inserts have the least impact on nylon.
Because glass fiber reinforced resin material has a small thermal expansion coefficient, it is more suitable for inserts. In addition, preheating metal insert before molding also has a good effect.

External stress

External stress here is mainly due to stress concentration caused by unreasonable design, especially at sharp corners.

External environment

Water degradation caused by chemicals, moisture absorption, and excessive use of recycled materials will deteriorate physical properties and cause cracks.

Two, insufficient filling

Main reasons for insufficient filling are as follows: insufficient resin capacity, insufficient pressure in the cavity, insufficient resin fluidity, and poor exhaust effect.

Improvement measures

As improvement measures, we can mainly start from following aspects:
①Extend injection time to prevent resin from flowing backwards before gate is solidified and difficult to fill cavity due to short molding cycle.
②Increase injection speed.
③Increase mold temperature.
④Increase plasticizing temperature of resin.
⑤Increase injection pressure.
⑥Expand gate size. Generally, height of gate should be equal to 1/2~l/3 of wall thickness of product.
⑦Gate is set at the place where wall thickness of product is the largest.
⑧Exhaust groove (average depth 0.03mm, width 3~5mm) or exhaust rod is set. It is more important for smaller parts.
⑨Leave a certain buffer distance between screw and injection nozzle.
⑩Choose low-viscosity grade materials.
Insufficient filling can sometimes produce wrinkles and pockmarked surfaces or shrinkage pits. Reason for this defect is essentially same as that of insufficient filling, but degree is different. Therefore, solution is basically same as above method.
Especially for resins with poor fluidity (such as polyoxymethylene, PMMA resin, polycarbonate and PP resin, etc.), more attention is needed to increase gate and appropriate injection time.
In addition, attention should be paid to uniform wall thickness in design, wall thickness should be reduced as much as possible in places such as reinforcing ribs and convex columns.

Three, overflow

Focus of overflow treatment should be mainly on improvement of mold. In terms of molding conditions, you can start to reduce fluidity.

Improvement measures

Specifically, following methods can be used:
①Reduce injection pressure.
②Lower resin temperature.
③Choose high-viscosity grade materials.
④Reduce mold temperature.
⑤Grind mold surface where flashover occurs.
⑥Use harder mold steel.
⑦Improve clamping force.
⑧Accurately adjust joint surface and other parts of mold.
⑨Increase mold support column to increase rigidity.
⑩Determine size of different exhaust slots according to different materials.

Four, warpage, deformation

Warpage and deformation of injection products 
Warpage and deformation of injection products are very difficult problems. It should be solved mainly from aspect of mold design, but adjustment effect of molding conditions is very limited.

Improvement measures

Causes and solutions of warpage and deformation can refer to following items:
① When deformation is caused by residual stress caused by molding conditions, stress can be eliminated by reducing injection pressure, increasing mold temperature and making mold temperature uniform, increasing resin plasticization temperature, or using annealing methods.
② When poor demolding causes stress and deformation, it can be solved by increasing number or area of push rods and setting demolding slope.
③When cooling method is not suitable, cooling is uneven or cooling time is insufficient, cooling method can be adjusted and cooling time can be extended. For example, cooling circuit can be placed as close to deformed place as possible.
④For deformation caused by molding shrinkage, mold design must be revised. Among them, the most important thing is to make product wall thickness consistent. Sometimes, as a last resort, we have to measure deformation of product and trim mold in opposite direction for correction.
Resins with greater shrinkage are generally crystalline resins (such as polyoxymethylene, nylon, polypropylene, polyethylene and PET resins, etc.), which deform more than non-crystalline (such as PMMA resin, polyvinyl chloride, polystyrene, ABS resin, etc.). In addition, since glass fiber reinforced resin has fiber orientation, deformation is also large.

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